As a production method of a metallized ceramic substrate, the co-firing method (simultaneous firing method) and the post-firing method (sequential firing method) are commonly known. The co-firing method is a method in which to form a metal paste layer on a non-fired ceramic substrate precursor called a green sheet, thereby produce a metallized ceramic substrate precursor, and then fire this. In this method, the green sheet and the metal paste layer are fired simultaneously.
The post-firing method is a method in which to form a metal paste layer on a ceramic substrate obtained by firing a green sheet, thereby produce a metallized ceramic substrate precursor, and then fire this. In this method, the green sheet and the metal paste layer are fired sequentially.
Both methods enable a metallized pattern to be formed on a ceramic substrate, and the resultant substrate is mainly used as a substrate for mounting an electronic component. As for the substrate for mounting an electronic component, further enhancement of precision and fineness of the metallized pattern is required as the component to be mounted becomes smaller. At present, such requirements cannot be met by the conventional production methods.
For example, in a case of forming wiring by the co-firing method, the green sheet easily shrinks unevenly at the time of firing; and when a square-shaped green sheet is sintered for instance, the sheet is caused to shrink slightly in a way that the central portion of each side of the square warps inwardly, and consequently the substrate deforms into a star-like shape. Therefore, when a number of metallized patterns in the same shape are formed on one green sheet, the patterns inevitably have a slightly deformed shape depending on the area where they are formed.
On the other hand, in a case of forming a metallized pattern by the post-firing method, a conductive paste is directly applied onto a ceramic substrate and dried, and then fired to thereby form the metallized pattern. When the conductive paste layer is fired, it shrinks in its thickness direction; however, it hardly shrinks in it planar direction. Therefore, the problem that the shape of the pattern changes depending on the position as seen in the co-firing method does not occur.
However, in the post-firing method, even if the metal paste is applied in accordance with a shape of an intended circuit pattern, the metal paste sometimes runs or bleeds before being fired. Consequently, refinement of the pattern is hindered. This is because when the metal paste “runs” or “bleeds”, a short circuit is sometimes caused between the wirings on the resultant metallized ceramic substrate, leading to degradation of the reliability thereof. In general, the post-firing method only enables, at best, formation of a metallized pattern wherein the gap (usually also called a space) between the metallized patterns is about 50 μm.
In order to solve the problems of the running and bleeding of the conductive paste as seen in the above post-firing method, the following suggestions have been made. Patent Document 1 suggests a method for forming a circuit, wherein an adhesive photosensitive resin layer is formed on a heat-resistant substrate; a circuit pattern is exposed to eliminate the adhesiveness of the exposed part; thereafter particles to form a circuit are contacted and adhered thereto; and the resultant is fired.
Patent Document 2 suggests a production method of a ceramic circuit substrate, wherein by using a conductive paste, a first circuit is formed on a resin film coated with a mold release agent; this is transferred to a surface of a ceramic substrate coated with a thermoplastic resin; and the resultant is fired.
Patent Document 3 suggests a production method of a ceramic circuit substrate, wherein a copper conductor paste is printed onto a substrate made of a ceramic and is fired to form a copper conductor circuit; thereafter the bleeding part of the copper conductor appearing on the edge of the circuit is dissolved to be removed; and the edge is treated sharply.
In addition, Patent Document 4 suggests a production method of a metallized ceramic substrate, wherein a ceramic paste layer is formed on a ceramic substrate, and a conductive paste layer is formed on the ceramic paste layer, to produce a substrate precursor; and the substrate precursor is fired.